Fruition of multimedia contents is a need which has recently become increasingly felt by the users, and will tend to increase even further in the future.
For example, many years ago a television signal included two components: an audio component and a video component. The teletext component was then added. More recently, especially for television programs which are broadcast via satellite, a plurality of audio components, each corresponding to a different language, have been included. Stereoscopic (or “3D) television programs are now beginning to gain ground; in simple words, in order to improve the three-dimensional effect perceived by the viewer, slightly different sequences of images are provided for the right eye and for the left eye.
Multimedia contents are generally produced for transmission through electric and/or optic signals; if the multimedia content changes, e.g. because new components are added, the transmitted signal will change as well; therefore, receiving such modified signals will require changes to the receivers owned by the users, if such changes are feasible, or else the users will need to purchase new receivers.
It must also be considered that, since many users are often not interested in viewing “improved” multimedia contents, it is important to ensure that such users can continue to view “non-improved” multimedia contents; for example, those users who are not interested in “3D television” must still be allowed to watch “2D television”.
In this regard, the DIOMEDES project financed by the European Union has proposed and studied the possibility of broadcasting a “3D content” through two “delivery channels”, i.e. a first channel using DVB technology (for the “basic content” that can be viewed by everybody) and a second channel using IP technology (for the “additional content”).
This project, however, has not specifically tackled the following aspects:                synchronization of the signals transmitted over the two “delivery channels”,        signaling for the signals transmitted over the two “delivery channels”;        
and has generically stated that telecommunication standards already included mechanisms which may be used for such purposes.
As far as synchronization is concerned, the DIOMEDES project has proposed to use the video PID's [Packet Identifiers] of each view, the audio PID's of each track, and the PID of the PCR [Program Clock Reference], and to generate the PTS's [Presentation Time Stamps] and the DTS's [Decoding Time Stamps] by starting from the same clock; such information has not however been conceived for establishing a synchronization between different information streams, and therefore it does not ensure any accuracy at all; moreover, since the information streams follow different and unpredictable paths (especially as concerns the Internet-transported stream), it may happen that one or both of them undergo multiplexing and/or re-coding operations and thus lose track of the common time reference originally given by the fact that they were two “elementary streams” of the same “transport stream”.
As far as signaling is concerned, the DIOMEDES project has proposed to use the signaling mechanisms described in the ETSI TS 102 809 V1.1.1 standard; this means that the “basic content” or “primary stream” must necessarily be transported with DVB technology, because this standard and the signaling thereof specifically refer to that technology.